This invention relates to a method of designing multi-conductor electric cables (both single-phase and multi-phase) which create a very weak external magnetic field, and to the structure of such cables per se.
Scientific research and investigation of influence of continuous exposure to existing environmental alternating electromagnetic field, which have been completed to date, have arrived at very significant conclusions. For example, it has been acknowledged that epidemiological evidence points to human health hazards in exposure to ambient alternating electromagnetic field environments exceeding 0.2 xcexcT. A dose-dependence of childhood leukemia is suggested for power frequency fields in the range 0.2-0.4 xcexcT. Assessment of the ambient magnetic environment in these studies at sites near power transmission and distribution lines has generally not taken account of much higher but more focal fields in the immediate vicinity of operating devices in the home and workplace. Resulting risk estimates may thus underestimate the true exposure levels from all sources. Although largely neglected in the emphasis on magnetic field bio-effects, there is also a body of laboratory evidence relating biologically significant effects, particularly in cerebral tissue calcium binding, to electric field exposures in the range 10-100 V/m. It must be emphasized that epidemiological studies do not rule out effects of electromagnetic fields on cancer risk, even large ones. This is because of limitations in exposure assessment and undoubted misclassification of exposure, as well as the absence of truly unexposed subjects.
The Regulations of the Israeli Electricity Law stipulate in Section 31 that, at medical sites where bio-potential measurements are provided (such as emergency departments in hospitals, ECG or EMG laboratories or the like) electric cables are to be screened to avoid or diminish interference caused by electrical equipment. Section 31 of the Regulations states that in such locations the maximal allowed value of magnetic induction be 2 *10xe2x88x927 to 4*10xe2x88x927 T (i.e. 2 to 4 mGs).
Screens which are utilized for protection from the excessive magnetic field are usually capable of reducing external magnetic field intensity by about 10-30%. Such a screen is effective for protecting the cable from ambient magnetic fields, but it decreases only slightly the external magnetic field created by the electric cable itself.
It should be mentioned that an electromagnetic field created in the vicinity of a current conducting cable may have a harmful effect on precise electronic instruments, computers and communication devices. In the prior art, attempts have been made to improve transmission capability of a multi-wired electrical conductor by reducing its coefficient of additional losses.
RU 2025014 describes a three-phase current cable for supply of electric energy users in three-phase circuits with frequency up to 10 kHz. The cable contains phase current conductors (for example, A,B,C), wherein each of the current conductors is in the form of a pair of parallel connected wires. The pairs of wires A,B,C are placed opposite to each other relative to the center of the conductor.
SU 1836766 discloses an electricity supply system, having three-phase current conductors with phases made in the form of parallel connected wires set symmetrically relative to a central wire. As explained in the patent specification, when a three-phase current passes through the current conductors, two equal oppositely directed magnetic fluxes are formed and smaller counter EMF""s (electromotive forces) are induced in the cable; the inductive resistance is thereby reduced together with the coefficient of additional losses. The system is declared to have an increased transmission capability.
It should be emphasized, that both of the above-mentioned technical solutions are focussed on achieving a minimal internal magnetic interaction in the multi-phase conductor for improving its transmission capability, and the goal is gained by providing a symmetrical structure of parallel connected wires of different phases.
U.S. Pat. No. 3,675,042 (Merriam) entitled xe2x80x9cApparatus for power transmission utilizing superconductive elementsxe2x80x9d, discloses a long electrical power transmission line utilizing superconductivity in which each conductor includes a superconductive portion and a normally conductive portion having high thermal conductivity with the two portions being in electrical and thermal contact along substantially their entire lengths. The conductors are in the shape of thin wires to provide a low internal magnetic field and permit high current densities. The conductors are connected in pairs into a plurality of direct current circuits which in turn are connected to one another in parallel and are arranged in a plurality of circular clusters to further minimize the internal magnetic field and which may be selectively connected between a power source and one or more loads.
In such an arrangement, the conductor material operates at or near zero degrees absolute. As the conductor radius increases, for constant current density, the magnetic flux density increases linearly in accordance with the equation:   B  =            I              2        ⁢        π        ⁢                  xe2x80x83                ⁢        r              ·    μ  
where:
B=magnetic flux density
r=radius of conductor
xcexc=magnetic permeability
I=conductor current
j=current density
There thus exists a critical magnetic flux density for superconductive material, beyond which the material ceases to superconduct. For this reason, as described at col. 6, lines 70ff and recited in the claims, the diameter of the conductor must be limited to no more than about 2 mm.
Each superconductive core is surrounded by copper cladding to quench fire if core loses superconductivity and gives rise to heating of the core. Thus, the actual distance of adjacent cores is significantly increased by the diameter of the copper cladding, leading to an increased external magnetic field.
U.S. Pat. No. 3,675,042 is thus directed to minimizing the internal magnetic flux density without regard to the external magnetic field. In contrast thereto, the invention is directed to minimizing the external magnetic field.
British Patent Publication No. 2 059 670 describes a high voltage cable for a three-phase power supply system, comprising six phase conductors each of the same cross-sectional area arranged symmetrically around a central null or protective conductor. The phase conductors are connected together in oppositely-situated pairs at their ends. GB 2 059 670 has as an objective the requirement to obtain voltage symmetry at the end of the cable and to limit the losses in a 3-phase line at higher frequencies. No suggestion is made to reduce the external magnetic field.
It is the two-fold object of the present invention to provide a method for designing a single-phase or a multi-phase electric cable having a very weak external magnetic field and, correspondingly, to provide a novel structure of such cables.
According to one aspect of the invention, the above object can be achieved by a method of designing a single- or multi-phase electric cable capable of conducting current through insulated conductors and creating a weak external magnetic field, the method comprising the following steps:
(a) assembling at least one of said conductors from two or more insulated sub-conductors to be connected in parallel, wherein the sum of cross-sectional areas of the sub-conductors is equal to a design cross-sectional area of said conductor, and wherein the sum of currents to pass through the sub-conductors is equal to a given current to pass through said conductor;
(b) arranging said conductors in the cable in such a manner that each of said sub-conductors is adjacent to a conductor or a sub-conductor associated with either a different phase or a different current direction; and
(c) ensuring a predetermined minimal strength of the external magnetic field by checking the following condition for the sum of magnetic moments:             ∑              i        =        1            N        ⁢          xe2x80x83        ⁢                            M          .                _            i        =  0
where N is a total number of the magnetic dipoles, and i is a number of a particular dipole.
The insulated sub-conductors may have cross-sections of a circular, rectangular or any other shape.
Needless to say, that according to the Kirchhoff""s Law the sum of all currents passing through all conductors and sub-conductors of the cable must be equal to zero. It is understood to those skilled in the art that the cable should be designed according to its operational conditions.
Preferably, the sub-conductors and conductors assembled and arranged according to the above definition should be placed in the cable as close as possible to one another. It is readily understood that technical limitations will be imposed by the design voltage, by quality and thickness of the electrical insulation, as well as by the cross-section of the wire.
As an option, the method may also comprise a step of twisting the arranged conductors in the cable.
A specific calculation step can be applied to the above method for ensuring that the above-described configuration provides a predetermined minimal strength of an external magnetic field.
For single-phase cables the calculation step includes arranging a number of so-called magnetic dipoles from currents passing via the mentioned sub-conductors and conductors, dividing the dipoles into groups, determining a value and direction of magnetic moment of each of said groups, and adjusting the arrangement of said conductors and sub-conductors in such a manner that the sum of magnetic moments of component dipoles in each of said groups is substantially zero.
In order to better understand the principle of the proposed approach, the following terms will be acknowledged herein below:
A dipole is a pair of currents having equal values and opposite directions and passing via a pair of adjacent wires (whether being a non-divided conductor and a sub-conductor, or two sub-conductors) in the cable.
A magnetic moment M of a dipole is a vector which can be defined as follows:
{dot over ({overscore (M)})}=xcexc0*{dot over (I)}*D*l0*{overscore (n)}0,
where:
xcexc0 is the magnetic permeability of vacuum,
I is the value of one of the equal and opposite currents in the dipole,
{dot over (I)} indicates the phase of alternating current
D is the distance between the parallel wires in the dipole,
l0 is an elementary length of the wire being one unit of length,
{overscore (n0)} is an elementary vector being perpendicular to the surface where the elementary lengths of two wires of the dipole are located; the vector n0 can be carried in parallel to itself, its direction thus defined according to the right gimlet rule.
Using the above definitions, the following condition of designing a single-phase cable with a plurality of sub-conductors can be written down:             ∑              i        =        1            N        ⁢          xe2x80x83        ⁢                            M          .                _            i        =  0
(where N is a total number of the dipoles, and i is a number of a particular dipole).
It has been found by the inventor, that even a pair of dipoles arranged in the cable according to the above described rules enables to achieve a significant reduction of the external magnetic field. The greater the total number of sub-conductors (or the greater the number of the formed pairs of dipoles in the formed multidipole), the weaker will be the external magnetic field created at any predetermined distance from the cable. It has been found that the larger the number of parallel wires and the smaller the distance therebetween, the higher is the degree of attenuation of the external magnetic field. In a case of an infinite number of wires spatially mixed in the cable, no external magnetic field would be created, like in the case of a coaxial electric cable.
Moreover, the longer the distance from the center of the cable, the sharper will be the character of attenuation of the external magnetic field.
In other words, the method comprises the step of adjusting the degree of attenuation of the external magnetic field by selecting a number of sub-conductors for assembling said conductors of the cable.
For example, by applying the above mentioned method to a single-phase cable, one may obtain a degree of attenuation of the magnetic field of many hundreds or thousands, or even more at an exemplary distance of 50 cm from the center of the cable (!), (compared to a very weak attenuation of about 10 to 30% which can be reached by screening of equivalent conventional cables).
For multiphase cables the additional calculation step may include calculating a resulting magnetic field created by all conductors and sub-conductors in the cable and adjusting a number, cross-section and configuration of the sub-conductors in the cable to obtain maximal decrease of an external magnetic field in the vicinity of the cable. In such a case, the magnetic flux density in the center of the cable is usually essentially equal to zero, which factor might be helpful for correct designing of the inventive cable.
In terms of magnetic moments, the calculation step may comprise determining magnetic dipoles formed in each phase conductor of the multiphase cable and a neutral wire, and arranging the dipoles to satisfy a system of equations wherein each of said equations is built for a specific phase conductor of the cable:                                                         ∑                              n                =                1                            N                        ⁢                          xe2x80x83                        ⁢                                                            M                  .                                _                            Rn                                =          0                ;                                                                    ∑                              P                =                1                            P                        ⁢                          xe2x80x83                        ⁢                                                            M                  .                                _                            sp                                =          0                ,                        ⋯                                                              ∑                              q                =                1                            Q                        ⁢                          xe2x80x83                        ⁢                                                            M                  .                                _                            Tq                                =          0                ,                  where          :                    
RS, . . . Txe2x80x94symbolize conductors of different phases of a multiphase cable;
N, P, . . . Qxe2x80x94symbolize total numbers of sub-conductors in each of the phase conductors R, S, . . . T, respectively;
n, p, . . . qxe2x80x94each symbolize a specific number of a sub-conductor in the phase conductors R, S, . . . T respectively;
{dot over ({overscore (M)})}Tqxe2x80x94symbolizes a particular magnetic moment created by a current passing in a sub-conductor q of the phase conductor T and a corresponding part of the current in the neutral conductor, when present, or in another phase when no neutral conductor is present.
The preferred version of the method for designing the multiphase cable includes the step of assembling each of m single-phase conductors of the cable from n equal sub-conductors, and the step of arranging said sub-conductors in a circle so that an angle xcex1 between each two sub-conductors is about 360xc2x0/mxc2x7n, and an angle xcex2 between each two sub-conductors belonging to the same phase is about 360xc2x0/n.
The degree of attenuation of the magnetic field which can be achieved by applying the method to multi-phase cables depends on the construction of a specific cable (number of sub-conductors, their arrangement, etc.); if required, the degree may reach hundreds or thousands. It is understood, however, that complexity of the cable""s construction will put a certain limitation to the maximal decrease of the external magnetic field.
It is known to those skilled in the art that the stronger the external electromagnetic field created by a cable, the easier the penetration of any ambient electromagnetic field into the cable, thereby creating electric disturbances therein, jamming in transmission lines, etc. It can now be seen, that if the magnetic field around the inventive cable can be minimized, the cable will be less sensitive to any external magnetic field. Such a property is of special importance for sensitive electronic devices (such as precise measurement instruments, computers, TV-sets, etc) and for all high frequency electronic devices. A value of mutual inductance between a reference cable and a cable of interest may serve as a measure of sensitivity of the cable of interest to external magnetic field disturbances.
Calculations and measurements which have been undertaken by the inventors, has proven that the mutual inductance of any modification of the inventive cable is much smaller than that of an appropriate conventional cable, and that the inventive cables are significantly less sensitive to external magnetic fields. The inventive cables also have smaller self-inductance than the conventional cables, thus the voltage drop along a transmission line formed by the inventive cables will be decreased.
According to a second aspect of the invention, there is provided a single-phase or a multi-phase electric cable for conducting current through insulated conductors and creating a weak external magnetic field, wherein:
at least one of said conductors is assembled from two or more insulated sub-conductors to be connected in parallel, wherein the sum of cross-sectional areas of the sub-conductors is equal to a design cross-sectional area of said conductor, and wherein the sum of currents to pass through the sub-conductors is equal to a given current to pass through said conductor;
the arrangement being such that each of said sub-conductors is adjacent to a conductor or a sub-conductor associated with either a different phase or a different current direction.
It is understood that the arrangement wherein the conductor(s) and sub-conductors (or the sub-conductors only) are placed in the cable as close as possible to one another, is preferable.
It should be emphasized, that the present invention does not impose on the sub-conductors any requirements of symmetry, equal cross-sections or specifically stated distances therebetween.
With respect to multi-phase cables, the invention also allows that at least one phase conductor in the cable is not assembled from sub-conductors.
According to one embodiment of a single-phase cable without a neutral wire, one conductor thereof is assembled from two sub-conductors which are symmetrically placed near the other (non-split) conductor from its two diametrically opposite sides.
In accordance with an alternative embodiment of the single-phase cable, each of the two conductors thereof comprises two or more sub-conductors to be connected in parallel to each other.
In the preferred embodiment of a multiphase cable, each of its m phase conductors may be assembled from n equal sub-conductors, and the sub-conductors are arranged in a circle so that an angle xcex1 between each two adjacent sub-conductors is 360xc2x0/m*n, and an angle xcex2 between each two sub-conductors belonging to the same phase is 360xc2x0/n.
The multi-phase cable, according to one specific embodiment of the invention, may comprise a number of phase conductors each being assembled from two or more insulated sub-conductors being equal or non-equal in cross-section; said sub-conductors being mixed in the cable in a manner providing for a minimal external magnetic field.
It is known, that in the case of a symmetric load of a multiphase cable, current in the zero-wire (neutral wire) is absent. In this case, position of the zero-wire in the cable is not important. However, for cases where a multiphase cable is loaded non-symmetrically, it is preferable that the zero-wire is positioned in the center of the cable. It has been noticed that the inventive construction of the cable is especially helpful for reducing external magnetic fields created around such non-symmetrically loaded multiphase cables.